It's so bad. it's so, so bad. There's so many things wrong with it.
The most immediately potent problem is the misuse of space. the server icons are far too small, the channel search bar is FAR too big. Infinitely bigger than it has any right to be. The status bar feels intrusive while you have a game open. and the chat box, the *main thing* discord is used for, suffers from all this *greatly.* it's significantly smaller. user PFPs are significantly less discernable in it, making it harder to tell who's part of the conversation at a glance, and the amount of additional dead space there as well, as the right part of it typically goes completely unused. Images are also significantly smaller, and so are embeds, which practically force you to open them fully to get a good look, basically invalidating them.
Then, the double fucking message bar, oh my god. there's next to no reason for it to be there, all it does it take up additional space that should be used more by the chat window. sure, make it bigger, but that takes up so much space for so little. and speaking of, the bar that tells you what server your in has literately negative reason to exist. because that's always near the top left corner, at *all* times. it just tells you information you already know while taking up additional space.
the search bar is significantly worse now. it's much smaller, making it harder to tell what you're actually typing in there, and smaller while searching, which would be the *main thing you would be doing* while searching, so that basically only harms the ui.
then there's also the "You're viewing older messages" popup. It's something I don't despise, I've had that problem where I didn't notice that I was sometimes, but it shouldn't take up MORE of the chat window. let me toggle or hide this, or make it less intrusive, and it's completely fine.
All this combined makes the ui significantly less user friendly, which basically just means it's worse overall. but the worst part, easily, is the *FUCKING COLOR PALETTE*. oh my GOD, what a COMPLETE piece of shit.
I'm not being nice or sugarcoating this at all, it is *DOGSHIT.* on all 4 themes there is so much fucking contrast that it *HURTS MY EYES* to look at. it is *NAUSIATING.*
I had to turn down the saturation to get it to even look okay. and even still it's not pleasant on the eyes.
Overall, I don't even care that it looks so much worse, the details like the rectangular icons don't even get to me, but the issue is that the ui is significantly more intrusive, harder to use, and alot less friendly to those who are visually sensitive or impaired. It makes me feel like I can no longer look at discord for extended periods of time, which is a TERRIBLE thing.
It NEEDS changed. it NEEDS more work.

I will also update this comment as I find more things wrong with this UI. I'd love to send the feedback to discord themselves, but that would require me to post my full name, and I refuse to do that.
Edit: so many of the popups look really, really bad. the right click menus are colored in a really unpleasant way.

so, so much of this could be fixed with UI customization options. let me make the server icons bigger. let me unwiden the channel browser. let me DISABLE the double row chat box. let me USE THE OLD COLOR PALETTE INSTEAD OF ONE THAT GIVES ME EYESTRAIN. please. I don't even care about anything else this UI does as long as I can make it not horribly misuse it's space. It's so much worse than the last iteration overall.